1. Field of the Invention
The present invention pertains to a method and system for monitoring the operation of one or more gas-lift fluid production wells and improving production from such wells.
2. Description of the Related Art
It is not uncommon for the reservoir pressure in typical oil wells to be insufficient to cause a produced fluid to flow naturally from the well. In such situations, the produced fluid (usually a multi-phase fluid containing gas, oil and water) must be artificially raised to the surface, and the typical methods currently used to artificially raise the fluid are submersible or beam pump and gas-lift. Submersible and beam pumping as well as gas-lift are applicable to surface facility forms (onshore on platforms). Beam pumping is not applied to sub-surface well applications. In deep wells, beam pumping is not routinely used because the extensibility of sucker rods used for pumping deprives the pump of a sufficient stroke. In such cases, gas-lift is often used when there is sufficient gas-lift gas available. In gas-lift methods of production, a production tubing string is installed within the cased opening. Production is attained through this production tubing. The annulus outside the production string, but within the cased hole, is used as the downward path for communication of gas, which is used by the gas-lift equipment. The process consists of forcing (compressing) gas under high pressure into the annulus. At the gas-lift equipment, gas is introduced into the production or tubing string to reduce the density of petroleum liquid produced from a deep formation so that the liquid will rise in the production tube. Hence, gas-lift valves located in side pocket mandrels are installed at various elevations within production tubing, and are adjusted in depth to reduce hydrostatic pressure. The lower the gas-lift valve the more liquid is lifted in the well.
Even though the reservoir pressure may not be sufficient to raise the produced fluid to the surface it will normally be sufficient to support a column of fluid within the tubing. The lift gas may be injected continuously or intermittently depending upon the rate at which the produced fluid, under the action of the reservoir pressure, flows into the tubing. If the reservoir pressure is sufficient to maintain continuous flow into the tubing, continuous injection of gas will cause the produced fluid-gas mixture to continuously flow to the surface of the well. If the reservoir pressure is insufficient to cause the continuous flow of produced fluids from the formation through the casing perforations and into the tubing string, intermittent gas injection at appropriate times may be the only efficient method of gas-lifting the fluid to the surface. Intermittent injection of high-pressure gas into the tubing string will cause the fluid column, or slug, which has accumulated over some period of time, to be propelled to the surface almost as if the fluid constituted a cohesive mass.
Typical methods for determining the characteristics of production fluid gas include flowing multi-phase production fluid through a test separator. The test separator separates the multi-phase fluid. Single-phase meters are then used to measure each of the separated gas, oil and water streams. A drawback of using test separators is the enormous expense of weight support and space required associated with the test equipment its installation and operation, especially when located on offshore operations. In offshore production systems, the test separator is typically installed on a platform. However, technology in deep water production systems is moving toward requiring subsea placement of the separator where maintenance costs will be high.
Injection gas adjustments based on the averaged measurements severely impact production rates with respect to the actual flowing conditions of the well. A drawback of current gas-lift well methods is that control systems used to vary injection gas rates do not have timely data upon which to base control decisions. There are considerable dead time and time lag in the measurements that present difficult control problems. Accurate real-time measurements regarding the rate and type of produced fluid (e.g. gas, oil and water) and the flow regime is not available in the typical system for varying the injection gas characteristics. Typically, rate is the injection gas characteristic varied.
Under current methods, determining the characteristics of the produced fluid are based on measuring the output of multi-phase flow through a test separator with single-phase meters to measure each of the separated gas, oil and water streams. In the typical system, a production tube leads to a manifold which is switchable between the test separator and an output production tube. It can take anywhere from 4 to 12 hours to test produced fluid to obtain useful data values. These volumetric rate measurements are time averaged due to the time required to make the determinations. As a consequence, the values are average values. These late and averaged values make it impossible to determine real-time dynamic characteristics of the well being tested. Therefore the operator cannot make incremental or real-time adjustments to the injection gas in order to improve well production. Additionally, test separators are not generally placed on every well. A characteristic of the produced fluids can change over time more rapidly than the measurements are completed.
The present invention solves the problems identified above by providing an apparatus and method for improving production from a gas-lift well including determining certain properties of the produced formation fluid flowing through the production tubing without separating the fluid, and making injection gas adjustments based on real-time multi-phase measurements.
According to one aspect of the invention, a method of improving production from a gas-lifted production well system including one or more interconnected wells comprises injecting gas into a well to lift formation fluid from the well; measuring at least one characteristic of the formation fluid with a multi-phase flow meter while the fluid is in multi-phase form; determining a value of at least one physical characteristic from the measurement; and adjusting the injection gas characteristic based on the determined value, the adjustment tending to quickly improve well system production by elabling decisions to be made using real-time output values. In accordance with the invention, a system for monitoring the operation of a production well on gas-lift or similar artificial lift operation is provided wherein certain properties of the production fluid flowing through the tubing string are determined by sensing such parameters as pressure, density, flow rate, permittivity and conductivity. Signals related to values of these parameters are then sent to a controller used to adjust injection gas volumes to increase production of the fluid from the well.